The present invention generally relates to growth of epitaxial layers and more particularly to a method of growing a plurality of epitaxial layers having different properties on a common substrate in a continuous epitaxial process, suitable for manufacturing a semiconductor device such as a stripe-type laser diode or a so-called graded-index waveguide separate-confinement heterostructure (GRIN-SCH) laser diode in which an active layer is surrounded by clad layers.
Laser diodes are currently used extensively in various fields such as optical telecommunications, optical information storage and reproduction on and from optical recording media such as compact disks or laser disks, optical measurement of various quantities, control of various systems and the like.
In most of these applications, a so-called stripe-type laser diode is used currently in which light is confined in a stripe-shaped narrow active layer surrounded vertically and laterally by clad layers. By constructing the laser diode as such, efficient oscillation of the laser is realized as a result of confinement of light into such a narrow active region.
In order to confine the light in the active layer, it is necessary to form the clad layers having a low refractive index such that the clad layers surround the active layer. This means that the active layer is not only sandwiched by a pair of clad layers, one at the top and one at the bottom but has to be bounded also laterally by a pair of clad layers. Further, in order to improve the efficiency of carrier injection, it is desired that the electric current, or carriers, injected to the active layer is confined laterally such that the carriers are concentrated in a part of the clad layer in contact with the active layer. For this purpose, it is necessary to form the clad layer into regions having different conductive types.
Conventionally, formation of such clad layers laterally bounding the active layer cannot be made simultaneously with the formation of the active layer. Similarly, formation of a clad layer laterally divided into a plurality of regions having different conductive types cannot be made immediately before or after the formation of the active layer without interrupting the process of crystal growth. Thus, when forming a structure having an active layer surrounded vertically and laterally by clad layers in an epitaxial growing system for growing crystal layers epitaxially, such as a molecular beam epitaxial (MBE) crystal growth system, a partially completed laser diode is taken out from the system and is subjected to various processes such as mesa formation, insulator film deposition, ion implantation, impurity doping and diffusion, and so on. After these processes are completed, the laser diode is returned to the epitaxial growing system for further formation of epitaxial layers. As will be easily understood, the step of taking out the partially completed laser diode from the epitaxial growing system for performing additional processes is undesirable as such a step, and the other processes performed outside the epitaxial growing system, tend to introduce defects in the laser diode. Thereby, the yield of the product is decreased.
Meanwhile, there is a so-called GRIN-SCH laser diode having a thin active layer of gallium arsenide (GaAs) forming an isolated quantum well structure. In this laser diode, a pair of clad layers of gallium aluminium arsenide (GaAlAs) are provided so as to sandwich the active layer and the composition of the clad layer is graded such that the content of aluminium (Al) is low at a side of the clad layer in contact with the active layer while the content is increased gradually as the distance from the active layer is increased. As a result of such a construction, the decrease of the optical confinement factor, indicative of the efficiency of confinement of light in the active layer, is prevented and associated therewith, the construction of a laser diode having a low threshold current density becomes possible.
Conventionally, such a graded clad layer used in the GRIN-SCH laser diode has been formed by changing the flow rate of a source gas of Al with respect to that of gallium (Ga) together with the progress of crystal growth of the clad layer when the clad layer is grown by a metal-organic chemical vapor deposition (MOCVD) technique. When the clad layer is grown by MBE, on the other hand, the temperature of a cell for holding a source material of Al and the temperature of a cell for holding a source material of Ga are changed such that a desired compositional profile is obtained.
In the presently available technique of MOCVD or MBE, however, there is a problem in that, although these techniques provide a satisfactory control of the Al content as long as the composition is constant throughout the clad layer, accurate control of the flow rate of the source gases or the temperature of the source materials, to a degree sufficient to provide a satisfactory compositional profile, is extremely difficult as such a control of the growth is indirect and not performed directly at the substrate where the growth is made.